Metazoan development is characterized by the specialization of cells which, functioning in harmony, allow for the growth and survival of the organism as a whole. The ultimate goal of this research is to understand the components which regulate the spacial and temporal organization of a multicellular organism. Toward this goal, we propose to continue studies of 117 antigen, a surface protein apparently involved in cell cohesion of D. discoideum amoebae. The Specific Aims are directed toward tow complementary objectives. One is to define the antigen in biochemical terms and thus understand the features which contribute to its cell surface expression and function. The other is to understand the role of the antigen in morphogenesis. The specific Aims are to: 1. Characterize the PI-G tail of 117 antigen. 2. Examine the endogenous enzyme activity responsible for the release of 117 antigen and its physiological role. 3. Study 117 antigen developmental regulation and function. We have documented that 117 antigen possess a number of unusual modifications, including a glycolipid structure (PI-G tail) that anchors the protein to the cello surface. The biological significance of that structure is realized by the presence of an endogenous enzyme capable of releasing the antigen into the medium as part of the cells developmental cycle. This developmental cycle involves the cyclic expression of 117 antigen, underscoring its importance in morphogenesis. Continued studies of the antigen are expected to expand significantly our knowledge of how cells regulate surface levels of specific proteins and do so in a cyclic manner. This plasticity of expression reflects a more usual mechanism of regulation which may be fundamental to the morphogenesis of a multicellular organism.